The tiny plastic fragments most people swallow every day can alter the community of bacteria taken from the human gut, and some of the shifts resemble patterns already tied to colorectal cancer and depression.
That is the finding of the first study to use actual human stool samples, rather than mice or engineered cell lines, to test how microplastics interact with the microbiome.
It was presented at UEG Week 2025 in Berlin on October 7, 2025, and published in the journal Gut Microbes in 2026.
Researchers at the CBmed research center and the Medical University of Graz, Austria, grew gut microbiome cultures from stool samples donated by five healthy volunteers.
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They then exposed the cultures to five common microplastic types at doses reflecting everyday human exposure, as well as higher levels.
The plastics tested were polystyrene, polypropylene, low-density polyethylene, poly(methyl methacrylate) and polyethylene terephthalate, the kinds found in food packaging, bottles, textiles and household plastics.
What changed inside the cultures
The total number of bacteria stayed largely stable. But the mix shifted.
The team saw plastic-specific changes in four bacterial families, Lachnospiraceae, Oscillospiraceae, Enterobacteriaceae and Ruminococcaceae, most of them within the phylum Bacillota, one of the dominant groups of bacteria that live in the intestines.
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The cultures also grew consistently more acidic, a sign that the microbes were metabolizing differently.
Some of those shifts reflect patterns previously linked to depression and colorectal cancer according to United European Gastroenterolog, the researchers report, with Lachnospiraceae among the families involved.
The finding is an association, not proof that microplastics cause either disease.
It is, however, one of the first pieces of evidence gathered directly from human gut material rather than from animals or lab models.
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Levels of several metabolic byproducts moved as well, including valeric acid, 5-aminopentanoic acid, lysine and lactic acid.
These are small molecules that gut bacteria produce as they digest food, and they help shape everything from the immune response to communication with the brain.
Changing the environment
Because the bacterial count did not fall much, the researchers argue the effect is not simple die-off.
Instead, microplastics appear to change the environment enough to favor some microbes over others.
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Christian Pacher-Deutsch, the study's lead author and a doctoral researcher at CBmed and the Medical University of Graz, added that the exact pathways remain unclear, but several plausible explanations are emerging, according to Medical Xpress.
The exposure route is not exotic. Microplastics have been documented in tap water, seafood, table salt and the air we breathe.
They have already been detected in the human bloodstream and, as earlier work has shown, in nearly every human brain examined so far.
What this means in practice
The study is small. Five donors, cultures grown outside the body rather than a living intestine, and no clinical outcomes measured.
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It cannot say microplastics will make anyone sick. What it does say is that at exposure levels people encounter every day, the microbes that live inside us respond, and some of those responses look like signatures seen in disease.
Pacher-Deutsch stopped short of clinical advice but said reducing microplastic exposure where possible is a wise and important precaution.
Everyday steps often named by researchers include drinking filtered tap water instead of bottled water, avoiding heating food in plastic containers and cutting back on single-use plastics.
Anyone worried about digestive symptoms or a family history of colorectal cancer should raise it with their doctor rather than change their diet based on a single small study.
This article is made and published by Ida-Marie Palm Varbæk, who may have used AI in the preparation.